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casting with blowing glass

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Lou C
Posts: 2
Joined: Sat Mar 30, 2019 9:27 pm

casting with blowing glass

Postby Lou C » Sat Mar 30, 2019 11:30 pm

Hi everyone... I haven't posted in years, but I am in need of some wisdom, and decided to see what you thought.
I recently had the opportunity to make some of my own colors using SprucePine batch and various metal oxides in a high temperature kiln. I made two colors that I really liked, a golden yellow and a blue, and invested in enough batch and chemicals to make perhaps 400 lbs of the two.. I had some time with a furnace, melted the batch, and then formed billets by ladling the glass into a frame on a marver table, and then putting them into an annealer. The billets were beeeuuuutiful.
Then I tried to cast with this glass, and it was a disaster... extreme veiling and devitrification, with a lot of bubble formation at the mold wall. The shapes were 10" diameter spheres, solid castings, so the top temp hold time and annealing time were necessarily quite long. Smaller castings with other glass in the same kiln run were clear. (glass specifically made for casting.) ((all molds were plaster/silica of similar mix))
It seems that I overlooked one TINY detail about this glass. I have now been informed that batch for blowing is formulated with a goal to make glass of even viscosity over a wide range of temperatures. Casting glass batch is formulated (I am told) to resist de-vit and flow at a lower temperature. And I clearly did not use it as intended....

So my questions are these...
What is a typical formula for a blowing glass batch?
And a casting glass batch?
Are the fundamental ratios of glass former, flux and stabilizer different in the two?
Or is one a kind of subset of the other, but with additional additives to provide the altered characteristics needed for casting?
Is there any feasible additive (or set of additives) that can be added to batch made for blowing such that it will produce glass which is robust enough for casting?

The glass may not be a total loss, if I can find a threshold of heat work that it can tolerate that can still permit thin castings. I am testing for that now.
And one final request for input... Is the following analysis correct? Please correct/clarify it for me if you can.
blowing glass stays for days in a furnace, and resists cords and property changes over time, and yet does not perform well at the much lower temperature when it comes to components within the glass beginning to crystallize and cause devitrification. In hindsight I guess this is because at high temperature the heat is sufficient to keep everything molten and crystallization is not possible.. degradation happens over time by a slow loss of flux from the melt. When this same material is much lower in temperature it is in a temperature region where it's component elements can begin to crystalize out of the melt. Casting glass has added components that reduce the mobility of the materials that tend to crystallize so that the crystals do not form as readily and increases the complexity of the melt in a way that interferes with and slows the crystallization characteristic of de-vit.
Any thoughts would be helpful!!! Thanks.

Kevin Midgley
Posts: 633
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: casting with blowing glass

Postby Kevin Midgley » Sun Mar 31, 2019 12:13 pm

These are questions you could possibly have better answered if you contacted the professors at Alfred University or took a course there.
Devitrification is started by a 'seed' for lack of a better term which could be the tiniest imaginable speck of dust or chemical combo on or within the glass.
You might have inadvertently allowed your glass to become microscopically dirty.

also ask here:http://talk.craftweb.com/forumdisplay.php?daysprune=-1&f=4

Lou C
Posts: 2
Joined: Sat Mar 30, 2019 9:27 pm

Re: casting with blowing glass

Postby Lou C » Fri Apr 05, 2019 2:25 pm

Hi Kevin.. thanks for your response. Sorry I did not respond sooner, I thought I would get an email to let me know someone answered.
I would love to take a class at AU, but I would need to do it as a short intensive... my other obligations prohibit any regular class there. Do they offer that kind of course? I will look into contacting some of the faculty, but if you have a suggestion of a good source that would be very helpful. I am also looking into what I can find thru the AU library.
Maybe I am causing confusion be not being clear about what I did. I made the billets in a hot shop, then took them to a kiln room and kiln cast them from a flower pot reservoir into a plaster/silica mold. I did not ladle cast (except to form the original billets.)
I have done some prior research on de-vitrification. the literature calls the seed a 'nucleus' and the process of nuclei formation 'nucleation'. True de-vitrification is crystallization of some of the components of the melt.. i.e. the glass is becoming less glass-like, less vitreous, and is therefore de-vitrifying. The literature also says that there are two types... internal, where the components of the glass form nuclei and then grow crystals, and external, where the nuclei is an external contaminant, or material not in the melt, but in the environment, like particles from a plaster/silica mold for example. You talked about the glass becoming 'dirty'... and I wondered if you were talking about the external kind, above?
I admit it is possible I did inadvertently alter the glass or introduce something that might act as 'nuclei'.... (I have been taken by surprise before) but I don't think so, because that same kiln run had several molds in it, all of the same construction, and several types of glass, and it was only the glass I made that had the problem.. All of the glass was handled (cleaned and positioned in the kiln) in a similar way. I believe my problem has to do with the composition of the glass itself and the way it was made.
If this clarification triggers any additional thoughts, please let me know! I really want to figure out a way to do this...
Lou

Kevin Midgley
Posts: 633
Joined: Mon Mar 10, 2003 11:36 am
Location: Tofino, British Columbia, Canada

Re: casting with blowing glass

Postby Kevin Midgley » Fri Apr 05, 2019 7:30 pm

The glass you made had more heat work and thus was more likely to devitrify.


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